This invention relates to particle detection systems and, in particular, to a method and apparatus to suppress generation or propagation of stray infrared light within particle counting systems.
Conventional particle counters measure scattered light from particles traversing a sample area illuminated by a light source. In scattered light sensing systems, any background source light that is scattered by internal cavities of the particle counter that is not associated with light scattered by particles will decrease the ratio between the scattered light signal and the background light signal. If the internal cavities of the particle detector are highly reflective, the light reflected off the walls due to any off-axis re-circulating particles can also be reflected back into the particle detector and detected as particles. Light blocking particle detection systems measure the reduction of light intensity blocked by particles traversing the viewing area. In light blocking systems, light that is scattered from the particles and reflected back into the detector will decrease the ratio between a particle detection signal and background noise. The same is true for any light that is reflected around the sample area and not used to sample particles. Undesirable detection of such stray radiation can create particle detection signals that can mislead the particle detection analysis of such signals.
Conventional particle counters employ visible wavelength lasers for a light source and are often fabricated from aluminum that is then black anodized to suppress unwanted internal reflection. Common black anodizing entails creating a porous layer of aluminum oxide on the surface of an aluminum part, such as an internal wall of a particle counting system. The part is subsequently soaked in an organic dye to create the black xe2x80x9ccolor,xe2x80x9d and then the dye is sealed into the part. The color black is actually a mixture of a number of different dyes, each of which absorbs a portion of the visible spectrum.
New generations of particle detectors are using different light sources including diode lasers that emit light in the near infrared wavelength region. Particle detectors employing different wavelengths have been more prone to experience stray light problems.
An object of the present invention is, therefore, to provide a particle system or method that employs light including a nonvisible wavelength, such as IR or UV, and suppresses stray light.
Another object of the invention is to provide such a system or method that employs an ionic anodize process to treat appropriate system components exposed to light.
Applicant""s graduate school work with organic dye lasers led him to recall that it was very difficult, if not impossible, to create chemically stable organic dyes that would absorb or lase near infrared light. He thus suspected that stray light problems associated with new generation particle detectors might be caused by the dyes used in the anodize process because such dyes might not be sufficiently effective in the infrared spectrum. He also speculated that organic dyes that might be initially effective in the infrared and used in the anodize process might also be chemically unstable in infrared applications.
The appropriate components of particle detectors of the present invention are anodized with an ionic coloring agent, instead of a traditional organic coloring compound, deposited into the porous aluminum oxide. Such appropriate components include, but are not limited to, optically or electrically nonfunctional internal components whose surfaces are off-axis from the light generation and detection axes. Because the atoms of ionic agents are more tightly bound and are not characterized by many of the energy transfer mechanisms available to the conventional organic anodize dyes, an ionic coloring process provides anodized parts that are more spectrally uniform.
A preferred OPTICAL BLACK(trademark) ionic anodize process of the present invention applies a porous anodic oxide coating in a standard sulfuric acid anodizing bath (with or without organic additives), subjects the porous oxide coating to a modified alternating electric current to remove excess anodizing electrolyte and to allow the coloring solution access to the pores, and electrolytically colors the oxide coating in a low pH acid bath containing a tin salt, sulfuric acid, and organic additives. Alternating current carries additional tin salt to the base of the pores where the tin salt is reduced to metallic tin or tin oxide. Others metals or metallic salts could be employed.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.